Aberrant intracellular calcium ([Ca2+]i) homeostasis has been implicated in cardiovascular diseases (CVD) which contributes to endothelial dysfunction. Ca2+ acts as a second messenger and is known to regulate several cell functions. Despite the available evidence of a link between Ca2+ signaling and endothelial dysfunction, mechanisms underlying the development of vascular disease remain incompletely understood. Besides Ca2+, reactive oxygen species have also been implicated in endothelial cell (EC) dysfunction. A growing recognition exists of a link between elevated levels of mitochondrial Ca2+ and subsequent generation of mitochondrial reactive oxygen species (mROS) in cardiovascular diseases. Mitochondria shape cytosolic Ca2+ signals by sequestering Ca2+ through uniporter and transporters. Nevertheless, the causal link between mitochondrial Ca2+ overload and mROS production is poorly understood in the context of EC dysfunction. Recently identified mitochondrial Ca2+ uniporter complex molecules MCU and MICU1 are shown to regulate the mitochondrial Ca2+ uptake, however the pathophysiological role of these molecules in endothelial function has not been studied. Our new findings show that MICU1 gates the basal mitochondrial Ca2+ accumulation by regulating MCU. Further, silencing of MICU1 facilitates constitutive mitochondrial Ca2+ overload which subsequently elevates mROS and sensitizes cells to apoptotic stimuli. Accordingly, our central hypothesis is that MICU1 gates MCU pore activity limiting basal mitochondrial Ca2+ accumulation and ROS overproduction to preserve vascular integrity. The hypothesis was formulated based on our recent publication in Cell. The hypothesis will be tested using combination of molecular and cell biology, biochemical and advanced imaging technology, primary endothelial cells, genetically-modified animals and samples from human subject with coronary artery disease. Our proposal will address these issues via three specific aims: 1) Characterize the functional role of MICU1 in EC mitochondrial Ca2+ homeostasis 2) Study the role of MICU1 in EC signaling and function, and 3) Investigate the role of MICU1 in EC biology under pathophysiological conditions. Importantly, our investigations will uncover the role of MICU1 in mitochondrial Ca2+ homeostasis and mROS production in vascular endothelium. The expected outcomes from these studies will significantly shift the focus of EC signaling by elucidating that MICU1 plays a central role in limiting mitochondrial Ca2+ load and oxidative signaling. Such findings are expected to have an immediate impact through advancing the fields of EC Ca2+ signaling and oxidative stress with a strong likelihood that the information will provide new targets for therapeutic interventions in CVD.